The present invention concerns ignition systems in which a spark is generated in order to ignite an air-fuel mixture in the cylinder of an internal combustion engine. In these ignition systems, the ignition coil has a primary winding and a secondary winding. An emitter-collector circuit of a switching transistor is connected in series with the primary winding. A measuring resistor is connected in series with the emitter-collector circuit of the switching transistor. The current for controlling the switching transistor is furnished by a control circuit which has an input line connected, generally through a resistor, to a common point of the emitter-collector circuit of the switching transistor and measuring resistor. The control circuit further has an output line which supplies the control current for the switching transistor. The control circuit prevents further increases of the current through the primary winding of the ignition coil after a predetermined value required for sufficient energy storage for ignition has been reached.
In the known circuits, oscillations are created in the primary winding at the ignition coil, and therefore across the emitter-collector circuit of the switching transistor when the limiting process takes effect. These oscillations are particularly undesirable during diagnostic testing since an incorrect adjustment of the ignition system can result from incorrect measurements created by such oscillations.